It is known that an organic compound having a nitroxide free radical such as the nitroxide compound (2) is prepared by oxidizing a secondary amine having a steric hindrance with a peroxide. That is, the nitroxide compound (2) can be prepared by oxidizing a cyclic secondary amine having a steric hindrance of the formula (1): ##STR3##
wherein T, R, R.sup.1, R.sup.2, R.sup.3, R.sup.4 and n are the same as defined above with a peroxide.
However, when 2,2,6,6-tetramethylpiperidine is used as a cyclic secondary amine (1) and it is oxidized to obtain 2,2,6,6-tetramethylpiperidine-N-oxyl, industrially satisfactory results are not attained, for example, the yield of the desired product is low as understood from the result of Comparative Examples described below, and the reaction time is long.